1. Field of the Invention
This invention relates to a magnetically recorded information reading apparatus utilizing the magneto-optic effect.
2. Description of the Prior Art
A method of optical reading out of magnetically recorded information by the use of the magneto-optic Kerr effect is known. An optical system as shown in FIG. 1 of the accompanying drawings is generally used for electrical detection of such recorded information.
In FIG. 1, an incident light beam 1 emitted from a light source (not shown) such as a laser and collimated is made into a linearly polarized light by a polarizing plate 2 and passes through a half mirror 3 and a condenser lens 4 to a perpendicular magnetic recording medium 5 on which information is recorded as a variation in direction of magnetization. The reflected light beam 1' reflected by the perpendicular magnetic recording medium 5 corresponds to the direction of magnetization (upward or downward) of the perpendicular magnetic recording medium 5 and the plane of polarization of the light beam is subjected to rotations in opposite directions by the Kerr effect and reflected as an elliptically polarized light. For example, if the plane of polarization of the light beam reflected by the downwardly magnetized portion is subjected to a rotation of .theta..sub.K, the plane of polarization of the light beam reflected by the upwardly magnetized portion is subjected to a rotation of -.theta..sub.K. This reflected light beam 1' again passes through the condenser lens 4 and the half mirror 3 and is transmitted through an analyzer 6. The reflected light beam 1' containing the information recorded on the recording medium 5 as the rotation of the plane of polarization is subjected to a variation in quantity corresponding to said information by the analyzer 6. Further, this light beam passes through a lens 7 and is photoelectrically converted by a photodetector 8, and said information is read.
The angle of rotation .theta..sub.K of the plane of polarization by the Kerr effect is usually substantially 1.degree. or less, and considering that the Kerr rotation modulated component obtained by passing through the analyzer 6 is of a very minute amount, it is necessary that the azimuth (azimuth angle) of the analyzer 6 be set to such an optimum position that the SN ratio of the detection signal is maximum.
Thus, in the magneto-optical information reading apparatus according to the prior art, the azimuth of the analyzer 6 has usually been set to 45.degree. relative to the direction of polarization of the incident light beam to maximize the magnitude of the detection signal. However, the influence of noise superposed on the detection signal differs depending on the characteristic of a detecting system (a photodetector and an amplifier) and, taking such noise into account, a maximum SN ratio cannot always be obtained at the azimuth of 45.degree.. That is, in a photodetector having no current amplifying function, such as a PIN-photodiode, heat noise having no dependence on the azimuth of the analyzer is dominant, while in a photodetector having the current amplifying function advantageous in respect of the SN ratio (for example, a photomultiplier tube or an avalanche photodiode (ADD)), shot noise in which the average noise power is proportional to the quantity of light detected is dominant and, due to the fact that the shot noise has dependence on the extinction factor and azimuth of the analyzer, a maximum SN ratio has not been obtained when a photodetector having the current amplifying function is used in the conventional magneto-optical information reading apparatus wherein the transmission axis azimuth of the analyzer is 45.degree..